1. Field of the Invention
The invention relates to microemulsion fuel compositions, and especially to such compositions having improved stability. Microemulsion fuel compositions have been of considerable interest since the combustion characteristics of such fuels have been found to be considerably different from those of the unmodified base fuels. Differences in combustion have been attributed to the presence of low molecular weight immiscible compounds such as water or methanol in the fuel as well as to the structural changes which accompany micellization of the surfactants which have been employed. The beneficial combustion changes include decreased smoke, particulate, and NOx emissions, and increased combustion efficiency. Improved fire resistance has also been demonstrated for microemulsion fuels containing water.
Microemulsion fuels are clear, stable, two-phase dispersions which form on simple stirring under appropriate conditions. They are comprised of a continuous non-polar hydrocarbon phase and a discontinuous polar phase. Because of the small droplet size of the discontinuous phase (2 to 200 nanometers) these fuels appear to be clear, one-phase systems.
2. Description of the Prior Art
The effects of water or alcohol addition on diesel engine performance is reviewed in "Water and Alcohol Use in Automotive Diesel Engines", DOE/CS/50286-4, published September 1985 by J.J. Donnelly, Jr. and H.M. White. The techniques for introducing water or alcohol into the engines covered by this review included (macro) emulsification, blending, fumigation, and dual-injection. The introduction of water or methanol was found to reduce emissions of smoke and particulates 20-60% while moderately reducing or increasing emissions of hydrocarbons and carbon monoxide. The addition of water also reduced levels of NOx 10-50%. This held true for all methods for introducing the water or methanol, and is attributed to a lowering of combustion temperatures (due to lower specific heating values and the heat absorbed to vaporize the water or alcohol droplets), and to a "microexplosion" phenomenon (the dispersed droplets vaporize explosively, more effectively atomizing the hydrocarbon fuel during combustion).
Water or methanol are most advantageously introduced into combustion engines when they are dispersed in the hydrocarbon fuel as a microemulsion. Since microemulsions are clear, stable, and pre-blended (prior to being stored in the fuel tanks), there is no need for additional equipment on the vehicle (as would be required for the other methods) such as additional fuel metering systems (dual-injection), agitators inside the fuel tanks (to prevent separation of macroemulsion fuels), injection or fumigation devices, etc. At the same time, the water or alcohol is still introduced into the engine in the desired physical for i.e., as microscopically fine liquid droplets (albeit dispersed as micelles in the hydrocarbon), preserving the ability to vaporize in the desired "microexplosion" manner.
An excellent general treatment of the subject of microemulsion fuel compositions is "Microemulsion Fuels: Development and Use" ORNL TM-9603, published March 1985 by A.L. Compere et al. Again, the presence of water or methanol (in microemulsions) led to large reductions in smoke and particulates, with slight increases in hydrocarbons and CO emissions. Depending on the type of engine used and operating conditions, NOx emissions were moderately decreased or increased.
Research sponsored by the U.S. Army Fuels and Lubricants Research Laboratory investigated the effect of water-in-fuel microemulsions on the fire-safeness of combat fuels. Several reports by W.D. Weatherford, Jr. and coworkers (AFLRL reports Nos. 111, 130, and 145) document the effectiveness of microemulsion diesel fuels containing 1-10% water inreducing the flammability--fuel pols were either self-extinguishing following ignition, or could not be ignited by an open flame. The Army formulations were prepared with deionized water, and surfactants without the addition of alcohols as cosurfactants. If low levels (200-500 ppm) of dissolved salts were present in the water, stable microemulsions could be formulated only by substantially increasing the percentage of surfactants, or by increasing the aromatic hydrocarbon content of the fuel. Even then, the amount of water that could be incorporated into the fuels were reduced when salts were present.
Various patents have issued which relate to microemulsion fuel compositions and which specifically relate to compositions comprised of hydrocarbon fuel, water, various alcohols, and surfactants. U.S. Pat. No. 4,406,519 for example, teaches a microemulsion fuel comprised of gasoline, methanol, water and a surfactant blend having a hydrophilic-lipophilic balance value of 3 to about 4.5. U.S. Pat. No. 4,083,698 describes fuel compositions which are water-in-oil emulsions and which comprise a hydrocarbon fuel such as gasoline or diesel fuel, water, a water-soluble alcohol such as gasoline or diesel fuel, water, a water-soluble alcohol such as methanol, ethanol or isopropanol, and certain combinations of surface-active agents. U.S. Pat. No. 4,451,265 describes microemulsion fuel compositions prepared from diesel fuel, water, lower water-miscible alcohols, and a surfactant system comprising N,N-dimethyl ethanolamine and a long-chain fatty acid substance. U.S. Pat. No. 4,451,267 teaches microemulsions prepared from vegetable oil, a C1-C3 alcohol, water and a lower trialkyl amine surfactant. This patent teaches the optional addition of 1-butanol as a cosurfactant for the purpose of lowering both the viscosity and the solidification temperature of the microemulsion.
In said co-pending application Ser. No. 825,841, filed Feb. 4, 1986, stable microemulsions are provided which employ tertiary butyl alcohol as a cosurfactant. It would, however, be advantageous to provide stable microemulsions with methanol as cosurfactant rather than tertiary butyl alcohol for reasons of cost and availability.
Methanol has been tried as a cosurfactant alcohol for anionic surfactants with limited success. U.S. Pat. No. 4,083,698 (col. 5, lines 5-10) claim that stable compositions are not feasible using long chain fatty salts (e.g., ammonium or sodium salts of oleic acid) as the surfactant and a water soluble alcohol (e.g., methanol) unless an additional nonionic surfactant is present. Compere, Griffith and Googin, in a paper submitted for publication, succeeded in producing a w/o microemulsion using a fatty acid surfactant and methanol as cosurfactant but it was stable only within a 5.degree. C. temperature span. U.S. Pat. No. 4,451,265 claims that stable w/o formulations can be prepared using a fatty acid neutralized with dimethylethanolamine (DMEA) and a Cl-C3 alcohol as the cosurfactant. The examples in this patent used ethanol as the cosurfactant, and 1.05 moles of amine/mole of fatty acid surfactant; none of the examples used methanol as the cosurfactant. When we employed methanol as cosurfactant with fatty acids fully neutralized with DMEA, the water uptake was less than 2% in the final mixture.
The foregoing demonstrates the difficulties in dispersing water as in a fuel-continuous microemulsion when methanol is the cosurfactant for an anionic primary surfactant, in the absence of other classes of additional surfactants or cosurfactants, e.g., nonionics.
Nonetheless, it is highly desirable to use methanol instead of other alcohols as the cosurfactant. Methanol is much less costly than other alcohols, and is readily produced from both renewable resources (e.g., wood) and from fossil hydrocarbons (natural gas, coal, petroleum). For lowemissionss fuels, methanol has been shown to substantially reduce emissions, e.g., particulates, NOx, whereas other alcohols are not nearly as effective. In internal combustion engines, the presence of methanol reduces the tendency of the engine to "knock". Methanol also helps suppress the freezing point of formulations containing water.